The overall goal of this ancillary study is to determine if iron deficiency in frequent whole blood donors adversely affects their neurocognitive function. Over 15 million red blood cell units are donated annually in the United States. Despite fulfilling all requirements for blood donation, almost two-thirds of the women, and half of the men, who are regular blood donors are iron deficient. The overall intent of this study is to develop a chain of evidence linking iron repletion in iron-deficient donors to (i) increases of iron in specific brain structures to (ii) improved processing speed and memory with improved efficiency and connectivity in related functional networks by functional MRI to (iii) improved performance on neurocognitive testing. To this end, we will recruit from a prospective, double-blind, randomized, placebo-controlled parent trial of iron-deficient regular blood donors. In Aim #1, we will determine whether iron repletion of iron-deficient blood donors improves neurocognitive test performance. In Aim #2, we will determine whether iron repletion improves processing speed and episodic memory by altering activity and connectivity in specific functional networks at resting state. In Aim #3, we will determine whether iron repletion increases iron concentrations in specific brain structures. The neurocognitive impact of iron deficiency due to frequent blood donation has not been rigorously examined and this critical knowledge gap is addressed by this proposed ancillary study. Completing this study will provide definitive evidence for the presence or absence of iron deficiency-induced neurocognitive and neurological functional deficits in frequent blood donors. Our results will lead to improved policies and practices regarding iron supplementation, iron monitoring, and donation frequency.